1. Technical Field
Low noise amplification and low noise amplifiers with controllable gain are disclosed.
2. Description of the Related Art
In different types of radar, sonar and ultrasound systems a variable gain amplifier is employed for signal compensation. In all these systems a pulse is emitted from some type of transducer. Echoes from objects are detected by the transducer and the distance to the object is calculated as the pulse speed in the medium times the time from pulse emission to detection. However, as the pulse travels in the medium, the pulse is attenuated. Hence, the echo strength will be lower for echoes arriving a long time after pulse emission compared to echoes arriving early.
A variable gain amplifier (VGA) is used to compensate for this effect. It is controlled such that amplification is increased with time with the same amount as the signal is attenuated. In this way the relative signal power at the output of the VGA can be kept constant.
Previously, the VGA functionality is most commonly implemented with two different approaches. The most common approach is to amplify the signal initially. A variable attenuator is following the first amplification stage resulting in a variable gain function. A better solution can be implemented using current domain techniques. FIG. 1 shows a simplified schematic circuit diagram, 100, of a single ended implementation of the gain adjustment. The input signal to the VGA, 100, in FIG. 1 has form of a current, IIN. In practice the gain adjustment circuitry most often will be combined with a voltage to current converter, transconductor, to allow for a voltage input to the VGA.
The operation of the circuitry is as follows. The input current IIN is mirrored by transistors M2 to M6. The size of each transistor is designed relative to M1 by the scaling factors M=xn such that the current in each of the transistors M2 to M6 are xn times the current in M1, where xn is the scaling factor given for a transistor. Output currents from transistor M2 to M6 are summed into a load resistor RL, and the current gain is defined as the current flowing through RL divided by IN. The current from transistors M2 to M5 are connected through differential pairs, 102, which based on the control voltage VGAIN, either steers the current through the load resistor or directly to the supply rail. V1 to V4 are threshold voltages used to determine when each differential pair is switched on. Typically, V1 to V4 would be at different voltages with a few hundred millivolt between each tap. The operation of each differential pair, 102, will depend on whether each differential pair is source degenerated or not. The size of the resistors at the emitter of the differential pairs will determine the voltage range of VGAIN required to turn the differential pair, 102 completely on or off.
Assuming that a given input current is applied to M1 and that VGAIN is set to zero. Also assuming that V1 to V4 are located at increasing voltage potential with V1 at a few hundred millivolt. In this design, all current from M2 through M5 will be steered directly to the supply voltage. The current through M1 will be mirrored by M6 and will be flowing through RL resulting in a current gain of one (1) assuming ideal transistors with the scaling factor shown in the figure. If a dynamic signal is applied to IIN, the signal current will be amplified with unity gain.
If VGAIN is increased, part of the M2 current will start flowing in the load resistor RL, gradually increasing the current gain. As the differential pair above M2 is fully switched on, the increased current gain will be set by the sum of the scaling factors of M6 and M2, which in the case shown in FIG. 1 is two (2). At the time VGAIN reaches V2, part of the M3 current will also be steered through RL increasing the gain further. Hence, by proper dimensioning of threshold voltages and characteristics of the differential pairs 102, the gain can be smoothly adjusted in a range determined by the current mirror scaling factors. The embodiment in FIG. 1 shows a linear-in-dB gain which means that gain increases exponentially with control voltage. However, alternate characteristics may be achieved by different current mirror scaling.
The implementation in FIG. 2 illustrates a possible differential implementation of a currently available VGA. Transistors M1 to M4 form a traditional prior art differential voltage to current converter. The input voltage is applied between the terminals VINP and VINN. Transistors M1 and M2 act as source followers. The input voltage, VIN=VINP−VINN is buffered with a gain close to unity and applied across the input resistor RI. The current through RI, called the signal current IS, will therefore be proportional to the input voltage under ideal conditions, IS=VIN/RI. As there is only one current branch at each side of the input resistor, the signal current, IS, will also be conducted through the input transistors and transistors M3 and M4. The resulting current through M3 and M4 is therefore a fixed bias current with the signal current IS on top. The currents through M3 and M4 are mirrored in inverting and non-inverting side circuitry respectively. The differential pairs are either steering the current through the inverting side load resistor or to rail whereby the resulting gain is adjusted. The number of current sources in the mirror can be set arbitrarily depending on required gain control range.